docs: def cli(env, volume_id, sortby, columns):
      """List block storage snapshots."""
      block_manager = SoftLayer.BlockStorageManager(env.client)
      snapshots = block_manager.get_block_volume_snapshot_list(
          volume_id,
          mask=columns.mask()
      )
  
      table = formatting.Table(columns.columns)
      table.sortby = sortby
  
      for snapshot in snapshots:
          table.add_row([value or formatting.blank()
                         for value in columns.row(snapshot)])
  
      env.fout(table)